Multiple andreev reflections spectroscopy of superconducting LiFeAs single crystals: Anisotropy and temperature behavior of the order parameters

The superconducting state of LiFeAs single crystals with the maximum critical temperature $T_c\approx17\,$K in the 111 family has been studied in detail by multiple Andreev reflections (MAR) spectroscopy implemented by the break-junction technique. The three superconducting gaps, $\Delta_\Gamma=(5.1$–$6.5)\,$meV, $\Delta_{\text{L}}=(3.8$–$4.8)\,$meV, and $\Delta_{\text{S}}=(0.9$–$1.9)\,$meV (at $T\ll T_c$), as well as their temperature dependences, have been directly determined in a tunneling experiment with these samples. The anisotropy degrees of the order parameters in the $k$ space have been estimated as $<8$, $\sim12$, and $\sim20\%$, respectively. Andreev spectra have been fitted within the extended Kümmel–Gunsenheimer–Nikolsky model with allowance for anisotropy. The relative electron-boson coupling constants in LiFeAs have been determined by approximating the $\Delta(T)$ dependences by the system of the two-band Moskalenko and Suhl equations. It has been shown that the densities of states in bands forming $\Delta_\Gamma$ and $\Delta_{\text{L}}$ are approximately the same, intraband pairing dominates in this case, and the interband coupling constants are related as $\lambda_{\Gamma\text{L}}\approx\lambda_{\text{L}\Gamma}\ll \lambda_{\text{S}\Gamma}$, $\lambda_{\text{SL}}$.